Firefox and Chrome trigger the request's `onerror` handler when a
timeout occurs. Safari triggers the `ontimeout` handler.

This can be observed by making a request to an unroutable address:

    var xhr = new XMLHttpRequest()
    xhr.onload = console.log.bind(console, 'loaded')
    xhr.onerror = console.log.bind(console, 'errored')
    xhr.ontimeout = console.log.bind(console, 'timeout')
    xhr.open('GET', 'http://10.255.255.1')
    xhr.send()

Fixes #294.

From https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest#getAllResponseHeaders():

> Returns all the response headers, separated by CRLF, as a string, or null if no response has been received.

I discovered this on Android, once when `xhr.getAllResponseHeaders()` evaluated to `null`.

Handle cases where `self` isn't defined

Exercise both polyfill and native `fetch` in test suite

When exercising the polyfill in Firefox, the invalid JSON parsing error
will cripple the test suite, causing cascading failures in Mocha HTML
output as well as cause failures in unrelated tests.

The code fix was proposed #157 but we decided not to pull it since it's
a Firefox bug and Firefox now ships with native fetch anyway. Instead,
we avoid running that test as part of our polyfill test suite.

Mysteriously, it doesn't implement it.

This is opposed to checking for native ArrayBuffer because the polyfill
will only define `arrayBuffer()` methods if Blob is available.

Before, in browsers that had native `fetch` implementation the polyfill
code was never exercised by the test suite. This ensures that both
implementations are exercised.

If someone just cloned this project, they might be tempted to
`script/server` and try to run the test suite immediately. That won't
work, because they have to run `make` first to obtain all npm and bower
dependencies. However, they would get a blank HTML page which would be
confusing because it's not immediately apparent what they did wrong.

Now, an error message is rendered that suggests you should run `make`.

Summary: The process of extracting a body from a passed in body object
(described here: https://fetch.spec.whatwg.org/#concept-bodyinit-extract)
creates both a byte stream and a corresponding `Content-Type` value. These two
values are used in the constructor of `Request` and `Response` objects to
initialize the body and to set an initial `Content-Type` header.

This change makes `Body._initBody()` return the `Content-Type` value
corresponding to the type of the object passed in, and to set that header in
the corresponding `Request` and `Response` objects that are being constructed.

There's a little bit of a dance I need to do in Request to keep track of where
the body is coming from, because we're only supposed to be setting the
`Content-Type` header if we're getting the body from the options passed in, not
if we're copying it from another `Request`. See step 32 of
https://fetch.spec.whatwg.org/#dom-request. (This was caught by one of the
tests. Yay tests!)

I added some corresponding tests to make sure this works.

Test Plan:
I'm a little bit confused about how to run the tests normally, but this is what
I did:

 - Run `make lint`, see that everything passes
 - Start up the test server by running `node script/server 3900 &`
 - Visit http://localhost:3900/test/test.html and
   http://localhost:3900/test/test-worker.html in Chrome and see that all of
   the tests pass.

From what I can tell, running the tests in Chrome just used the native fetch
objects, so to test the polyfill I added `window.fetch = null` in
test/test.html before it includes '../fetch.js', and added `self.fetch = null`
in test/worker.js before it imports '../fetch.js'. Then:

 - Visit http://localhost:3900/test/test.html and
   http://localhost:3900/test/test-worker.html in Chrome and see that all of
   the tests pass.

Reviewers: @jeresig

https://travis-ci.org/github/fetch/jobs/88862914

Allow making a POST request with an ArrayBuffer body

Run Sauce Labs CI for pull requests

[ci skip]

Closes #209, fixes #185

https://fetch.spec.whatwg.org/#dom-response-clone

Remove moot `version` property from bower.json

Every request now goes through general-purpose `saucelabs-api` command
which, by default, dumps the data in line-based format suitable for
parsing on the command line. If `--raw` is passed, raw JSON is returned
instead.

Disable Travis' own Sauce Labs integration

Use absolute URL in Response.redirect test

Avoids an issue where native implementations resolve and expand the URL.